I had a Manton 10/9 bore ML, it had been honed out perhaps several times and was very thin by the muzzels. I shot it with 3 dram/1.125 ounce 12 gauge loads (and "safety tested" both barrels with a 6 dram load in each barrel) and it did just fine. If it's a percussion gun it should work great on grouse, if a flintlock not so good. I say this because of the lag time between pulling the trigger and the gun going bang.
Steve

Approach life like you do a yellow light - RUN IT! (Gail T.)